Back pain, a significant source of morbidity in our society, is directly related to the pathology of the intervertebral disc. At present, the treatment of degenerative disc disease consists of therapies, both conservative and invasive, that are aimed at symptomatic relief. Recent advances in the understanding of tissue repair and stem cell biology have served to identify possible new approaches to rebuild diseased/degenerate tissues. Accordingly, the goal of the investigations described in this proposal is to develop a "tissue engineering" based approach to disc therapy. The Specific Aims of the investigation are: 1. To examine the differentiation of MSC into a nucleus pulposus-like cell; to evaluate the commitment and fate of these engineered cells in an in vitro cell and organ culture system. MSC will be maintained under conditions that enhance differentiation into a nucleus pulposus phenotype. At selected time intervals, differentiation of the disc cells towards a phenotype consistent with that of the nucleus pulposus will be assessed. We will test the hypothesis that the unique micro-environment of the intervertebral disc serves to enhance the differentiation of MSC to cells displaying a nucleus pulposus-like phenotype. 2. To isolate a notochordal population of cells from the nucleus pulposus; to examine the impact of these cells on the commitment of MSC to the nucleus pulposus phenotype. We will examine the impact of notochordal cells, isolated from the nucleus pulposus of the young rat, on the differentiation of MSC and the formation of a nucleus pulposus-like tissue. We will co-culture notochordal with the pre-differentiated MSC prior to their insertion into the nucleus pulposus cavity. We will test the hypothesis that cells of notochordal origin stimulate the expression of the nucleus pulposus phenotype. 3. To assess whether transplanted MSC can be used in vivo to repopulate the rat nucleus pulposus. Suspended in an injectable hydrogel, we will surgically transplant pre-differentiated MSC into the nucleus pulposus cavity of normal rat discs in vivo. We will evaluate the vitality of the cells and their creation of a nucleus pulposus-like tissue. Outcomes from this study will validate the in vitro studies performed in Specific Aims 1 and 2 and permit us to test the hypothesis that MSC could differentiate into nucleus pulposus-like cells in vivo. In the long term, if successful, this approach to therapy will entail using patients' own cells for disc regeneration.